It is well-known that the incorporation, in various manners, of a burnable absorber with nuclear fuel rods, which enables the use of excessive amounts of fuel in a reactor during the initial life of the fuel, can extend the life of the fuel rods. In some instances, the burnable absorber is mixed directly with the fuel and integrated therewith, while in other instances, a burnable absorber coating may be applied to the surface of fuel pellets, or discrete forms of a burnable absorber may be interspersed between conventional fuel pellets, or otherwise located within the cladding for the nuclear fuel. In U.S. Pat. No. 3,427,222, for example, a fuel rod is comprised of a tubular cladding that contains fuel pellets which have a fusion-bonded coating on the surface of each pellet, the coating comprised of a boron-containing material that functions as a burnable absorber.
It has also been proposed to provide cladding materials such as zirconium-based alloys that have various coatings or barrier means on the inside wall of the tubular cladding to protect the cladding from attack by constituents released from the nuclear fuel during operation of a reactor containing the fuel rod. As examples of such coatings or barrier means which cover the full surface of the cladding, U.S. Pat. No. 4,022,662 describes a cladding in connection with a separate unattached metal liner comprised of stainless steel, copper, copper alloys, nickel or nickel alloys, the liner disposed between the cladding and the fuel material. A diffusion barrier of chromium or chromium alloy is also disposed between the cladding and the metal liner. In U.S. Pat. No. 4,045,288, a composite fuel element cladding is described which comprises a zirconium or zirconium alloy substrate having a metal barrier of 1-4 percent of the wall thickness formed from niobium, aluminum, copper, nickel, stainless steel and iron, and an inner layer of stainless steel, zirconium, or a zirconium alloy metallurgically bonded on the inner surface of the metal barrier.
Also, it has been proposed to provide a burnable absorber, such as a boron-containing compound, directly in connection with the cladding material. U.S. Pat. No. 3,019,176, for example, discloses a fuel element that has a mixture of particulate fissionable material and a metal hydride moderator disposed in a matrix of a radiation resistant metal which is encased in a container. The matrix is bonded to the metal hydride and to the container to form an integral fuel element. U.S. Pat. No. 3,103,476 discloses the incorporation of a burnable absorber, such as boron, into the cladding of a nuclear fuel element. The boron is added to the cladding, which is preferably stainless steel, but may be zirconium or other material, in an amount of 200-1000 parts of natural boron per million parts of cladding material and homogeneously dispersed throughout the cladding. U.S. Pat. No. 3,625,821 describes a nuclear fuel element that has a zirconium or zircaloy cladding tube, with the inner surface of the tube coated with boron which is a burnable absorber. The boron is dispersed, as finely dispersed particles, in a matrix of nickel or other retaining metal. In Canadian Pat. No. 682,057, there is also described a fuel element where a cladding contains a burnable absorber. An outer layer of the tube may be of a corrosion resistant metal or alloy, and an inner layer is preferably comprised of the same material but contains, in addition, boron that may be a mixture or dispersed in the metal. The stated advantage over cladding containing dispersed boron is that boron, as part of the inner layer, is not contacted by coolant.
It is an object of the present invention to provide a composite nuclear fuel cladding that has a burnable absorber integrally incorporated therein.